The alloys have to have a stud diameter of 4x100, with a centre bore of 59.1mm (I think) or bigger. If the centre bore is any bigger, you have to use spigot rings to ensure your alloys fit properly. As for the actual size of the alloys, they can't be any less than 13", because they won't fit over the calipers. They can be pretty much any width though, but you want anything mega-wide, you'd have to get bolt on arches because the tyres will stick out too far. I think the biggest tyre size you can go for are 205s, otherwise they hit the suspension struts (unless you get wheel spacers).
But you also have to get different sizes depending on the width and size of the alloys, and how low the car is. I mean, my alloys are 15" x 8", and the car has been lowered 90mm. I can only tyres with a 45 profile, but because the tyres are deep dished and spaced out, can go for tyres as wide as 215 (as long as they have a 45 profile.
So if you were to lower the car by, say, 50mm (which certainly seems to be the norm around here), you could have tyres with pretty much any profile.
It just depends on how big you want the alloys to be, whether or not you want them to have a slight dish on them, how big you want the tyres to be, and how much you want to lower the car by.